The present invention relates to the field of printing and, in particular, it concerns a method for reducing variations in print density particularly suitable for use in inkjet printers.
It is known that output from inkjet printers often suffers from problems of non-uniform print density. In other words, regions of the output which are intended to appear a uniform shade actually exhibit variations in shade. This is caused by a number of factors including: lack of uniformity of drop size fired by different nozzles of the print head, and varying precision of drop position from different nozzles which result in uneven coverage of the substrate.
Any problem of non-uniformity of the printed output which is caused by features of the print head will appear in the printed output as a regular pattern corresponding to the movements of the print head over the substrate. One well known technique for reducing the visibility of these cyclic variations is multi-pass printing in which the print head passes over each region of the substrate to be printed two or more times with overlapping swaths. Although this technique tends to attenuate the variations and increases the spatial frequency of the variations, it does not achieve uniformity of output.
A number of approaches have been proposed for providing print quality feedback to modify operation of a print head. Of most relevance to the present invention is U.S. Pat. No. 5,798,773 to Hiramatsu et al. which discloses an apparatus and method for correction of density unevenness. The apparatus employs a reader to identify unevenness in a printed calibration pattern and then performs an unevenness correction. This correction is described as being implemented xe2x80x9cby correcting the drive signal (signal duration or voltage) of the required nozzles of the recording headxe2x80x9d (column 5, lines 24-26), thereby varying the size of drops ejected by the inkjet nozzles in selected locations.
While the approach of Hiramatsu et al. is theoretically correct, implementation of this approach is in most cases complicated and over costly. Specifically, a typical inkjet printer has thousands, and often tens of thousands, of nozzles operating simultaneously. The hardware requirements to enable selective adjustment of either the actuating voltage or the pulse duration for individual nozzles are typically prohibitively expensive.
There is therefore a need for a method for reducing variations in print density which would at least partially compensate for unevenness of output from a print head without requiring the complicated hardware modifications required by the Hiramatsu et al. technique.
The present invention is a method for reducing variations in output print density from an inkjet printer.
According to the teachings of the present invention there is provided, a method for reducing variations in print density in a printed output on a substrate resulting from defective nozzles of a print head, the method comprising: (a) obtaining a print density distribution for at least part of the print head, the print density distribution being indicative of at least one region of reduced print density due to defective nozzles; (b) assigning output reduction factors between 1% and 99% to a plurality of nozzles which are positioned within the print head so as to contribute to print density within the at least one region; (c) receiving data corresponding to an image to be printed; and (d) applying drops of ink to the substrate while passing the print head over the substrate, wherein numbers of ink drops applied to the substrate along lines traveled by each of the plurality of nozzles are increased as a function of a corresponding one of the output reduction factors.
According to a further feature of the present invention, each of the output reduction factors is generated as a function of print density over a region covered by a plurality of nozzles.
According to a further feature of the present invention, each of the output reduction factors is generated as a function of print density as measured by scanning a sample output at a resolution lower than the printing resolution of the print head.
According to a further feature of the present invention, the numbers of drops are increased by printing selected dots along the lines twice using two distinct nozzles during two passes of the print head.
According to a further feature of the present invention, the numbers of drops are increased by modifying screen values in a portion of a screen associated with locations to be printed by the plurality of nozzles.